1. from an area of higher water conc. to an area of lower water conc.
osmosis
GCSE
the movement of water
from an area of higher water conc.
to an area of lower water conc.
across a selectively permeable membrane

2. concentrated solution
dilute solution
more water
concentrated solution
less water
selectively
permeable
membrane
solute
molecule
water
molecule
hypertonic
solution
hypotonic
solution

3. At AS level we refer to solutions
as being hypertonic, hypotonic or isotonic
a hypertonic solution has more solute and less water
than a neighbouring solution
we say the solution on the RHS
of the diagram on the last slide
is hypertonic to the solution on the LHS
a hypotonic solution has less solute and more water
than a neighbouring solution
we say the solution on the LHS
of the diagram on the last slide
is hypotonic to the solution on the RHS

4. isotonic solutions on either side of a selectively permeable membrane
contain the same concentration
of solutes and water

5. osmosis animation

6. this is the tendency of a cell
water potential y
this is the tendency of a cell
to take in water by osmosis from pure water
across a selectively permeable membrane

7. water potential is a pressure so units = kilopascals kPa
Pure water has a water potential of ZERO
it is unable to take in water from a solution by osmosis
All solutions have a water potential below zero
i.e. negative water potentials
water will diffuse out of cells with a high water potential
into cells with a lower water potential
e.g. from a dilute solution with = -3kPa to a cell with = -300kPa y y

8. water particles attach to the solutes to form hydration shells
in solutions
water particles attach to the solutes
to form hydration shells
SOLUTE
Water particles forming
a hydration shell
so solutes reduce the ability of water to move freely
and to diffuse through the solution
therefore the water potential of all solutions
is less than pure water i.e. less than zero

9. the more concentrated a solution is (more solute)
the more negative the water potetial is
this is because there is more water
bound to the solute particles
and there are fewer to move freely in the solution
concentrated solutions are therefore more likely
to take in water by osmosis

10. a water potential of -10kPa is larger than
Water potential / kPa
Size
Negativity
Ability to take in water
-600
higher/ bigger
less negative
less likely
-1000
lower/ smaller
more negative
more likely

11. water always moves from a higher to a lower water potential
i.e. from a less negative to a
more negative water potential
-96kPa Y
-96kPa
Y=-400kPa Y
-83kPa Y
-115kPa
Y=-600kPa

12. the POTENTIAL of a solution
solute potential y s
the POTENTIAL of a solution
to take in water
is always negative
This differs from water potential because water potential is affected by factors such as the available space in a cell.
A turgid cell will still have the potential to take in water, as it is more concentrated than pure water, but because it is turgid it may be unable to, as there simply is no space available.
That is why water potential is the tendency to take in water, it takes this into account.

13. concentrated solutions have many solute molecules
so have a low solute potential (more negative)
dilute solutions have few solute molecules
so have a high solute potential (less negative)

14. A non-turgid cell will have less pressure.
pressure potential y p
This is the effect of pressure on the solution. A turgid palnt cell will have a high pressure on its cell wall due to the full contents of the cytoplasm and vacuole.
A non-turgid cell will have less pressure.
This therefore influences the ability of the cell to take in water by osmosis.
pressure potential
is usually positive,
but can be zero

15. relationship between water, solute and pressure potentials= + y y y s p
cell

16. Calculate the water potential for each of the following cells and predict the direction of water movement A
Ys= -400kPa
Yp = 300kPa
Ycell = ……….. A
Ys= -500kPa
Yp = 100kPa
Ycell = ……….. A
Ys= -500kPa
Yp = 300kPa
Ycell = ………..
-100kPa
-400kPa
-200kPa B
Ys= -500kPa
Yp = 300kPa
Ycell = ……….. B
Ys= -600kPa
Yp = 200kPa
Ycell = ……….. B
Ys= -600kPa
Yp = 400kPa
Ycell = ………..
-200kPa
-400kPa
-200kPa 1 2 3

17. http://www. phschool. com/science/biology_place/labbench/lab1/watpot
animal cells

18. An animal cell in a hypotonic solution (more dilute) will gain water by osmosis because the water potential outside the cell will be higher (less negative) than inside. As the cytoplasm increases in volume it pushes against the cell membrane causing the pressure to increase. Eventually the cell membrane will rupture, a process called lysis or haemolysis.

19. hypertonic solution
hypotonic solution

20. When an animal cell is placed in a hypertonic solution (more concentrated) it will lose water by osmosis causing the cell to shrink; a process called crenation.

21. http://www. phschool. com/science/biology_place/labbench/lab1/watpot
plant cells

22. Plant cells in hypertonic solutions will lose water from the cytoplasm and vacuole by osmosis as the solution outside will have a lower water potential than inside.
As the protoplasm shrinks the pressure inside the cell decreases and the membrane starts to move away from the cell wall and causes the pressure potential to decrease.
When the membrane only just touches the cell wall the pressure potential is zero. This is called incipient plasmolysis.
Tissue containing cells which have lost water in this way are said to be flaccid and cause a plant to wilt.

23. The cells are described as plasmolysed and the cells are killed.
As water continues to be lost, the membrane tears away from the cell wall, only remaining attached at the plasmodesmata.
The cells are described as plasmolysed and the cells are killed.

25. Turgidity is important in plant support.
Plant cells in hypotonic solutions will increase in volume as water moves into the cytoplasm and vacuole by osmosis.
This pushes the cell membrane against the cell wall. Pressure builds up but the strong cell wall pushes back onto the cell contents, preventing it from bursting. The cell is firm and described as turgid.
Turgidity is important in plant support.

26. turgid cell Pressure of cytoplasm and vacuole pushing
against cell wall
Pressure of cell wall
pushing back against cytoplasm and vacuole
The water potential of a turgid cell is zero;
no more water can enter.
Therefore, from the water potential equation
the pressure potential must be
equal to the solute potential

27. cell at incipient plasmolysis
The pressure potential at incipient plasmolysis is zero;
the wall no longer pushes back on the cytoplasm and vacuole.
Therefore, from the water potential equation the solute potential must be equal to the water potential.